Basin-scale distribution and activity of deep-sea protists in the North Atlantic Ocean

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Little is known about the distribution and ecology of eukaryotic microbes of the deep sea water column. Most of these microbes are small heterotrophic flagellates that feed on bacteria, where biomass in turn is fueled by the input of dissolved and particulate organic material from the surface. This study seeks to understand the distribution of eukaryotic microbes (i.e., protists) in the context of large, basin scale variations in hydrographic and chemical properties. The main hypothesis is that the abundance and taxonomic composition of protists serve as sensitive indicators of the strength and type (particulate or dissolved) of input of organic carbon into the deep ocean system. Samples in vertical profiles targeting major water masses across the North Atlantic will be collected. In addition, deep sea samples will be retrieved under pressure and incubated at in situ pressure and temperature in four newly designed chemostat systems. These cultures will be sub-sampled under pressure and examined for nutrient concentration, as well as for the purpose of monitoring the abundance of both prokaryotes and protists in the chambers. Using the same pressure samplers in short-term incubations, the investigators will explore the activity of deep sea protists by investigating the proportion of actively feeding organisms on fluorescently labeled bacteria. They will enumerate deep sea protists using a combination of fluorescence in situ hybridization and traditional staining methods, and will support taxonomic classifications using electron microscopy. Semi-automated epifluorescence microscopy with image analysis capabilities will be used to scan major filter areas and probe for rare microbes that normally fall below detection limits of other methods. In laboratory experiments, the investigators will use the newly built culture system to study pressure effects of eukaryotic protists while simulating temperature and pressure changes that sinking particles are exposed to when they sink to the abyss. Intellectual merit: The deep ocean is the largest habitat on earth. Among the three domains of life (i.e., Bacteria, Archaea and Eukarya), the eukaryotic fraction has largely been ignored in the microbial realm of this environment. New molecular methods make it possible, for the first time, to not only accurately enumerate protists but also to classify them according to their taxonomic affiliation. The investigators have a powerful tool to better understand the role of protists in the deep sea and to use them as sensitive indicators for the level of input of organic material. Protists may also control bacterial populations on sinking particles and thereby influence remineralization rates. As the deep sea differs in many ways from surface water (high pressure, low temperature, scarce food resources, darkness), deep sea protists provide excellent model organisms to address major research questions in the areas of physiology, ecology and evolution. Broader impacts: This is a collaborative research effort between Old Dominion University and the Netherlands Institute of Sea Research. The exchange of technology and ideas will benefit both institutions and student training. Deep sea research provides a fascinating teaching tool for marine courses. The pressure chambers will be incorporated into the biological oceanography core course for undergraduate and graduate students at Old Dominion University in order to give them hands-on experience in the exploration of pressure effects on biological and biochemical processes. Biological processes are highly relevant for carbon storage in the deep sea. The ocean's capacity to take up and store large amounts of excess carbon from the atmosphere over long time periods is of utmost importance to society in an era in which greenhouse gases accumulate in the atmosphere and lead to climate change. Data of this research will be shared via the national Ocean Carbon and Biogeochemistry (OCB) data repository at Woods Hole and through the Integrated Marine Biogeochemistry and Ecosystem Research (IMBER) framework. We will contribute to a planned permanent exhibit on the role of marine microbes at Nauticus, a premier maritime museum and interpretative center in Norfolk, with the title "Invisible world - the realm of microbial oceanography".